I. Field of the Invention
The present invention relates to communication systems, particularly to a method and apparatus for performing handoff from a code division multiple access system to an alternative technology system.
II. Description of the Related Art
In a code division multiple access (CDMA) cellular telephone system or personal communications system, a common frequency band is used for communication with all base stations in a system. The common frequency band allows simultaneous communication between a mobile unit and more than one base station. Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high rate pseudonoise (PN) code. The high rate PN code is used to modulate signals transmitted from the base stations and the mobile units. Transmitter stations using different PN codes or PN codes that are offset in time produce signals that can be separately received at the receiving station. The high rate PN modulation also allows the receiving station to receive a signal from a single transmitting station where the signal has traveled over several distinct propagation paths.
In an exemplary CDMA system, each base station transmits a pilot signal having a common PN spreading code that is offset in code phase from the pilot signal of other base stations. During system operation, the mobile unit is provided with a list of code phase offsets corresponding to neighboring base stations surrounding the base station through which communication is established. The mobile unit is equipped with a searching receiver or element that allows the mobile unit to track the signal strength of the pilot signal from a group of base stations including the neighboring base stations.
A method and a system for providing a communication with the mobile unit through more than one base station during the handoff process are disclosed in U.S. Pat. No. 5,267,261, issued Nov. 30, 1993, entitled "MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM," assigned to the assignee of the present invention. Using this system, communication between the mobile unit and the end user is uninterrupted by the eventual handoff from an original base station to a subsequent base station. This type of handoff may be considered a "soft" handoff in that communication with the subsequent base station is established before communication with the original base station is terminated. When the mobile unit is in communication with two base stations, a single signal for the end user is created from the signals from each base station by a cellular or personal communication system controller.
Mobile unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the mobile unit. The Active Set is the set of base stations through which active communication is established. The Neighbor Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having a pilot signal strength of sufficient level to establish communication. The Candidate Set is a set of base stations having a pilot signal strength of sufficient level to establish communication.
When communications are initially established, a mobile unit communicates through a first base station and the Active Set contains only the first base station. The mobile unit monitors the pilot signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. When a pilot signal of a base station in the Neighbor Set exceeds a predetermined threshold level, the base station is added to the Candidate Set and removed from the Neighbor Set at the mobile unit. The mobile unit communicates a message to the first base station identifying the new base station. A cellular or personal communication system controller decides whether to establish communication between the new base station and the mobile unit. Should the cellular or personal communication system controller decide to do so, the cellular or personal communication system controller sends a message to the new base station with identifying information about the mobile unit and a command to establish communications therewith. A message is also transmitted to the mobile unit through the first base station. The message identifies a new Active Set that includes the first and the new base stations. The mobile unit searches for the new base station transmitted information signal and communication is established with the new base station without termination of communication through the first base station. This process can continue with additional base stations.
When the mobile unit is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. Should the signal strength corresponding to a base station of the Active Set drop below a predetermined threshold for a predetermined period of time, the mobile unit generates and transmits a message to report the event. The cellular or personal communication system controller receives this message through at least one of the base stations with which the mobile unit is communicating. The cellular or personal communication system controller may decide to terminate communications through the base station having a weak pilot signal strength.
The cellular or personal communication system controller upon deciding to terminate communications through a base station generates a message identifying a new Active Set of base stations. The new Active Set does not contain the base station through which communication is to be terminated. The base stations through which communication is established send a message to the mobile unit. The cellular or personal communication system controller also communicates information to the base station to terminate communications with the mobile unit. The mobile unit communications are thus routed only through base stations identified in the new Active Set.
Because the mobile unit is communicating with the end user through at least one base station at all times throughout the soft handoff processes, no interruption in communications occurs between the mobile unit and the end user. A soft handoff provides significant benefits in its inherent "make before break" handoff technique over conventional "break before make" techniques employed in other cellular communication systems.
New CDMA systems are typically initially deployed in areas with existing FM or other technology systems. The initial deployment of the CDMA system may be gradual and cover only a portion of an operating area covered by the original existing system. In such a case, as a mobile unit communicating in CDMA mode drives from the coverage area of the CDMA system to a portion of the system without CDMA coverage, a handoff from the CDMA system to the original system is needed in order to facilitate continuous communication. The process of mobile unit assisted soft handoff as described above is not possible between the CDMA system and the original system. The handoff from the CDMA system to the original system must be performed as a "break before make" hard handoff. When performing a hard handoff it is especially important to ensure that the handoff will be successful because a failed hard handoff typically results in a dropped call.
It is therefore the object of the present invention to provide a method and apparatus for performing handoff from a CDMA system to a system employing a different technology.
It is therefore a further object of the present invention to provide an inexpensive and reliable means of detecting entry into the coverage area of an alternate system.